Modem computing devices typically utilize some form of pointing device for interactions of the user(s) with the Operating System (OS) or Graphical User Interface (GUI). The Capacitive Touch Pad (CTP) is well established as the pointing device of choice for Laptop and Notebook portable computers, and other devices. Most laptop computers are produced with a Capacitive Touch-Pad being either a main pointing device or one of the two built-in pointing devices.
The Touch Pad pointing devices are quite popular in portable computers due to simplicity of operation and fair pointing performance. Touch Pad pointing devices based on the capacitance sensing also feature “pressure-less” operations, where the finger actually does not exert any force on the pad, and may in fact hover over the surface of the pad without touching it. This contrasts with other Pad-form pointing devices, such as Resistive Touch Pad units that rely on the deformation of the surface in order to produce an electrical contact at the point of touch and, ultimately, the coordinates for the point of contact. The resistive Touch Pad may require the use of a special stylus, a familiar tool for PDA devices.
When working in a system, the CTP is exposed to a variety of noise sources from inside of the device, such as main CPU clocks, memory signals, etc., as well as external sources, such as cellular phones operated in close proximity and voltages induced on the fingers of the user, which include interference from a power line (mains) at 50 Hz or 60 Hz. It is desirable to reduce the sensitivity of the CTP to all of these noise sources.
A typical office or home environment provides plentiful opportunities for generation of static electricity, with potentially harmful Electro-Static Discharges (ESD) to the electronics of the CTP, when the user is first touching the surface of the Capacitive Sensor. It is desirable to prevent misoperation and/or destruction of the CTP from ESD.
As the name implies, the Capacitive Touch-Pad relies on the measurements of the changes of capacitance in order to ascertain the position of the pointing digit.
A Capacitive Touch-Pad typically consists of the Sensor with an array of electrodes arranged into two groups perpendicular to each other, and the data acquisition and control circuit. More often than not, the Sensor and all of the requisite electronic components are combined into a single assembly, with the PCB (Printed Circuit Board), typically of a multi-layer construction, serving both as the circuits interconnect and the Sensor itself.
The electrodes are typically arranged in such a way that one group runs parallel to the X axis of the unit, and the second group runs parallel to the Y axis of the unit, with the electrodes in each group proportionately spaced across the face of the unit in, respectively, X and Y dimensions, for example, as illustrated in FIG. 6.
Previous investigators have pursued many ways to try to employ such an arrangement of electrodes and the circuitry required to extract the position information. While the electrodes may be implemented using complicated shapes and various methods to facilitate galvanic isolation between the groups of electrodes and between individual electrodes in each group, nearly all prior-art approaches have generally followed the structure depicted in FIG. 6.
When the finger approaches the surface of the Sensor, a local disturbance of the capacitance is created. The amount of disturbance is measured on the nearby electrodes, and the exact position between the electrodes is extrapolated.
Typically, one group of electrodes would supply excitation in the form of the trains of pulses, with one or more electrodes providing the excitation waveform, and the rest of the electrodes in the same group floating or tied to the constant potential. Excitation can also be generated in the form of controlled currents.
On the other hand, the same or the second group of electrodes would be used to measure the AC waveforms resulting from cross coupling between various electrodes, both due to Sensor construction and the proximity of the pointing digit.
It is possible to create a circuit where each and every electrode is capable of both supplying the excitation and measuring the AC waveform.
The extraction of the useful signal is typically carried with a synchronous detector, capable of suppression of noise and unwanted signals due to their phase difference or non-synchronicity to the excitation signal.
A Block Diagram for the Prior-Art circuits is shown in FIG. 9.
Depending on the construction of the Sensor, the shape and relative location of individual electrodes, the resulting cross-coupling waveform may be smaller, larger, or the same for various positions of the finger relative to the Sensor.
U.S. Pat. No. 6,222,528 by Gerpheide et al., describes a system where a common electrode is used to pick-up a cross-coupling signal from X and Y excitation electrodes as the means for measurement of the disturbance of the local capacitance due to proximity of the pointing digit, as shown in FIG. 10.
Such a system suffers from three major limitations.
1.) The sensing electrode as well as X and Y electrodes must all be exposed, or partially exposed, on the front surface of the touch pad, since the electrostatic flux lines must be able to pass between the pointing digit and each of the X, Y, and sensing electrodes. Since the front of the touch pad has only a finite area, this must be shared between all electrodes, potentially limiting the useful amount of capacitance change between the X or Y electrodes and ground, when the finger approaches the face of the touch pad.
2.) Thus the sensing electrode must be continuously exposed to electrical interference, over the whole front surface of the touch pad, potentially collecting the electrical noise.
3.) The response of the sensor to an approaching digit has an ambiguity in the readings. When the finger first approaches the face of the sensor, some of the cross-coupling flux is diverted to the finger, and via the body's capacitance, to the ground. Thus the signal becomes smaller. Then, as the finger further nears the front of the sensor, the cross coupling increases. It is not possible to deduce the true position of the pointing digit without complicated computations. In some situations it is not possible to determine the position at all.
The operations of the Capacitive Touch-Pad could also be described using terminology of “charge transfer”, “charge redistribution”, “differential capacitance sensing”, etc. A person skilled in the art will have no difficulty in converting, reciprocally, to the methods described above.
It should be appreciated that nearly all of the Prior-Art circuits incorporate either a multiplexer to connect various electrodes to a single measurement circuit, or a plurality of measurement circuits acting at the same time on various electrodes.
It should also be appreciated that nearly all of the Prior-Art circuits are implemented as a unique custom-designed Integrated Circuit (IC) that contains both the analog circuitry necessary for Data Acquisition from the Capacitive Touch Pad Sensor, and the digital circuitry, often in the form of microcontroller, for extraction and processing of digital position information, and for communications with the Host system. However, the provisions for the analog and digital portions of this IC are quite different, with independent and often conflicting requirements, which include the type of silicon process, number of masks and processing steps, and feature dimensions on the die. Therefore, the single-IC approach may or may not be the most cost-effective, and the circuits made up from several ICs, some analog, and some digital, may in fact have a lower cost.
Additionally, the design and fabrication of a single application-specific IC may incur significant time as well as manufacturing expense. Any changes or adjustments to the original design may sustain the same level of expenditure. Therefore, it is advantageous to utilize common components, available “off the shelf”, and it is desirable to be able to make changes and adjustments (such as dimensions or functionality of the CTP, for example), without going through the whole process of design and development of a custom IC.